The inelastic blunting of hemispherical contacts

Predictive modeling is an integral part of the "Intelligent Processing of Materials" (IPM) concept which combines "model based control" and "in-situ non destructive sensors" for processing cost effective, high performance Metal Matrix Composite (MMC) and advanced alloy components for future aerospace applications. Models for the densification behavior of MMC monotapes and powders rely on the accurate prediction of stresses required to cause surface asperity (or interparticle) contact deformation. In the past, indentation models have been used to develop contact mechanics relationships, where simple criteria relate mean stress at the contact to the effective strain (or strain rate) for various densification mechanisms (plastic yielding and power-law creep). These models have been shown to be in error for several reasons; (1) the deformation of MMC monotape asperities and metal powder contacts are not accurately described by self-similar indentation, (2) the role of work hardening during plasticity was ignored and (3) the increasing lateral constraint with deformation experienced by asperities/particles during consolidation was neglected. In the work reported here, the evolution of contact stress with the deformation and velocity during elastic and inelastic blunting of contacts is computed using the finite element method (and verified experimentally). The calculations have been performed for a wide range of power-law creep exponents (to simulate different material laws) and an inelastic blunting model obtained. Then, the results are expressed in the form of "effective" constitutive relations in a non dimensional form; the normalized mean contact stress and normalized effective strain (rate) are related through non dimensional coefficients that are functions of only deformation and strain rate sensitivity. Variation of contact radius with the deformation has also been obtained. The inelastic blunting model is then implemented in the macroscopic densification models of monotapes and powders and the fiber fracture model for monotapes. Previous "Hot Isostatic Pressing" (HIP) maps for the monotape and alloy powders have also been revised.